Certain research and manufacturing processes require the use of a process chamber with high vacuum. For example, in semiconductor wafer processing, vacuum is used during many thin-film deposition and etching operations, primarily to reduce contamination. In such processes, pumps capable of producing a “high vacuum” of 10−6 Torr or lower are useful to assure adequate pumping speed at process pressure, and to allow for a low base pressure for cleanup between steps.
Several currently-available vacuum pump configurations are capable of producing and maintaining a high vacuum. Roots vacuum pumps and hook and claw vacuum pumps include two precisely machined rotors that rotate with equal speed in opposite directions, trapping gas in an exhaust portion of the casing and forcing it to an exhaust port. Roots and hook and claw vacuum pumps are used as primary vacuum pumps and as backing pumps. Another vacuum pump configuration capable of maintaining a high vacuum is the turbo-molecular vacuum pump, which relies on the high-speed rotation of rotor vanes in close proximity to stator vanes to induce molecular movement of the gas.
In each of the above cases, the pump inlet may open directly to the process chamber or may communicate with a process chamber through a foreline. The vacuum processing exhaust path may include a backing pump in a downstream position in the vacuum processing system exhaust path for reducing a pressure drop across the primary pump. The vacuum processing system exhaust path may also include an abatement system for reclaiming, removing or neutralizing various components of the exhaust.
One problem frequently encountered in vacuum processing systems is the deposition of solids by components of the gases passing through the vacuum processing system exhaust path. In one exemplary process 100, shown schematically in FIG. 1, the process gases 110 used in the semiconductor manufacturing process are WF6, H2, NH3, B2H6, SiH4, and Ar. WF6 can react with H2, NH3, B2H6 and SiH4 to form film deposits of metallic tungsten, tungsten salts (e.g., ammonium tungstate), or fluoride salts (e.g., ammonium fluoride). The reaction may take place in the process chamber or in various locations in the exhaust path, including the foreline, high vacuum pump 120, or in a roughing pump or abatement devices. Pump exhaust gases 130 include inert or unreacted components of the process gases, such as Ar and unreacted WF6, as well as reaction products formed during deposit formation in the vacuum processing system exhaust path, such as HF, BF3 and SiF4.
Another mechanism that may cause deposits on surfaces along the vacuum exhaust path is the buildup of powders formed by gas phase nucleation. Gas phase nucleation results from reactions among gas phase components of the exhaust gas, resulting in a solid reaction product in powder form. That product may accumulate on surfaces with which it comes in contact. As used herein, the term “deposit” shall mean solids accumulating on surfaces in the exhaust path due to any physical mechanism, including film deposition, gas phase nucleation or other phenomenon.
However they are formed, the deposits adversely affect the performance of the vacuum pump and other devices in the vacuum processing system exhaust path. For example, solid metallic deposits on pumping elements of roots pumps and hook and claw pumps change the effective shape of the pumping elements, degrading the efficiency and effectiveness of the pump.
To address the problem of unwanted deposits on devices in the exhaust path, a carefully selected reactive gas may be injected upstream of the affected device. The reactive gas prevents the formation of deposits by reacting with the exhaust gas components.
In an example of such a technique used in the above-described vacuum process system, NF3 is injected into a Remote Plasma Source (RPS) mounted in the foreline or directly at the inlet to a Roots or hook and claw pump, by means of a tee connector. The RPS is intended to dissociate the NF3 into atomic fluorine which can then react with gas phase molecules to prevent deposition, or react with deposited solids to remove them. Alternatively, molecular fluorine or other fluorine-containing gases may be substituted for NF3. In some cases, molecular F2 alone may be used without prior RPS dissociation.
By providing a fluorine source in the foreline or at the pump inlet, the reactions that cause solid deposits are prevented or at least greatly diminished. The atomic fluorine provides a more readily available fluorine source, preventing, among other things, a reaction of WF6 with the other gases or hot pump mechanisms, and thereby preventing deposition of metallic tungsten in the vacuum pump.
The fluorinated gases discussed above are generally very costly. For example, the cost of NF3 in the above-described system can amount to 10% or more of the total cost of materials in a semiconductor manufacturing process. It is therefore in the interest of the practitioner to optimize the amount of reactive gas used. In some processes, the process gases are pulsed, in which case it is advantageous to pulse the fluorine source in synchronized fashion with the appropriate gas. In some cases it is desirable to maintain a flow of the fluorine source for some time after the pulse of process gas is complete.
In any case, it is desirable to have enough of the fluorinated gas present while avoiding an excess. One way to accomplish that is to experimentally pre-determine the correct ratio of fluorine source to reactive process gas. In many cases, however, the flow rate of the process gas may change, either due to aging of components or to deliberate process recipe modification. If that happens, the set point may no longer be valid, insufficient fluorine source will be delivered, and the deposition that was intended to be prevented will occur.
There is therefore presently a need to provide an improved vacuum process control incorporating a solution to the problem of accumulating deposits in the devices in the vacuum exhaust path, while controlling the cost of reactive gases used. To the inventor's knowledge, no such control is presently available.